In the past, various kinds of touch type operation input devices have been devised, which detect the touch position and the amount of depression during the touch when an operator touches an operation surface in a flat shape with a finger.
For example, Patent Literature 1 describes a touch type operation input device having a structure in which a flat panel shaped pressure-sensitive sensor and a touch panel for detecting the touch position are in close contact with each other in an overlapping manner. In this touch input device, the touch position is detected by the touch panel. The amount of depression is detected by a pressure-sensitive sensor formed separately from the touch panel and placed on the touch panel in an overlapping manner.
When piezoelectric elements are used as the pressure-sensitive sensor in the touch type operation input device explained above, it is necessary to, e.g., apply a reference voltage of a predetermined bias to a piezoelectric element and use an amplification circuit such as an operational amplifier to amplify the output voltage in the piezoelectric element in order to detect the amount of depression.
Patent Literature 1: Japanese Patent Laid-Open No. H 5-61592